Recent years have seen increased use of small-sized memory cards, such as SD memory card, as recording media for cell phones, digital cameras, and so on. As device performance is enhanced, e.g., as the number of digital camera pixels increases, there has been increasing demand for memory cards with further increased capacity.
FIG. 17 illustrates exemplary stacked mounting structures with semiconductor chips conventionally used for satisfying the above demand. In the example shown in the figure, two stacked mounting structures 100a are arranged on one motherboard 104. Each stacked mounting structure 100a has semiconductor chips 105 mounted on one side of their respective boards 107. The board 107 and the semiconductor chip 105 mounted thereon are paired, and there are four pairs stacked on the motherboard 104 to form one stacked mounting structure 100a. The pairs of the board 107 and the semiconductor chip 105 are electrically connected by solder balls 108 provided between the boards 107 that are vertically adjacent to each other. In addition, the board 107 disposed at the bottom and the motherboard 104 are also electrically connected by solder balls 108 provided therebetween.
Also, there have been proposed stacked mounting structures having a semiconductor chip disposed on each side of boards as shown in FIG. 18 (see, for example, Japanese Laid-Open Patent Publication No. 2002-207986). In the example shown in the figure, two stacked mounting structures 100b are arranged on one motherboard 104. Each stacked mounting structure 100b has a semiconductor chip 105 mounted on each side of boards 107. The board 107 and the two semiconductor chips 105 mounted thereon are paired, and there are two pairs stacked on the motherboard 104 to form one stacked mounting structure 100b. An insulating sheet 125 is disposed where two semiconductor chips 105 face each other.
Also, there have been proposed stacked mounting structures each configured by folding a flexible board with semiconductor chips mounted thereon (see, for example, FIG. 1 of Japanese Laid-Open Patent Publication No. 2001-217388, and FIG. 1 of Japanese Laid-Open Patent Publication No. 9-199665).
The above-described conventional stacked mounting structures have size problems that are to be solved to satisfy the demand for further increased capacity. Concretely, the size of memory cards as typified by the SD memory card is defined by standards. Accordingly, a memory card (stacked mounting structure) with increased capacity cannot be achieved unless either the capacity per mounted semiconductor chip or the number of stacked semiconductor chips is increased.
Here, in order to increase the capacity per semiconductor chip, it is necessary to increase the recording density of the semiconductor chip. In this case, a new semiconductor design/fabrication process has to be developed, imposing high cost burden. In addition, it takes some time between the development phase and actual distribution to the market for use in memory cards. That is, there are significant problems with fabrication cost and lead time. As such, an attempt to increase the capacity per semiconductor chip in order to increase the capacity of the memory card leads to a significant cost increase.
Accordingly, from the perspective of cost, a desirable method would be to increase the capacity by increasing the number of stacked semiconductor chips. However, in the case of the above-described conventional stacked mounting structures, due to restricted size of the memory card, the number of stacked semiconductor chips cannot be increased unless each pair of the semiconductor chip and the board is reduced in thickness. In this case, it is necessary to further reduce the thickness of the board while maintaining structural strength and insulation properties. Therefore, a new design/fabrication process has to be developed, causing problems similar to those in the case of increasing the recording density of the semiconductor chip.
Also, in the case of the above-described conventional stacked mounting structures, a number of boards having the semiconductor chip(s) mounted thereon have to be stacked with conductive materials provided therebetween for connection, and therefore it is difficult to ensure satisfactory connecting quality for lamination. Accordingly, for that reason as well, the number of stacked semiconductor chips is limited.
As well, in the case of the above-described conventional stacked structure using a flexible board, the flexible board is required to cover the entire surfaces of the semiconductor chips for insulation, resulting in increased cost and limited number of laminations due to the thickness of the board.
The present invention has been made in view of the aforementioned problems, and an object thereof is to provide a stacked mounting structure in which the number of semiconductor chips that can be stacked within the same volume is greater than conventionally.